Vibratory screening with a peripheral support base

ABSTRACT

A chip screen for separating over and undersized wood segments from the desired size chips incorporates a screen plate assembly which is mounted on a supporting base by means of corner bearings attached to the periphery rather than the bottom of the assembly, thereby lowering the center of gravity of the unit and providing a more stable unit. An eccentrically mounted stub shaft engages a socket on the bottom of the screen plate assembly to impart a circular translational movement to the screen plate assembly. The socket is made oversized and a resilient insert interposed between opposing surfaces of the socket and stub shaft so that the shaft may be centered with respect to the corner bearings without radial constraint and undue tensile and compressive stresses thereby avoided. A pair of counterweights are attached to the drive shaft and positioned longitudinally and circumferentially thereof such that the centrifugal forces generated by the counterweights and the screen plate assembly produce a dynamically balanced system of forces on the machine as a whole.

United States Patent 1 Christensen Sept4, 1973 VIBRATORY SCREENING WITH A [73] Assignee: The Black Clawson Company,

Hamilton, Ohio 22 Filed: Aug. 31, 1970 211 App]. No.: 68,357

[52] US. Cl 209/332, 209/366, 209/415,

74/86 [51] Int. Cl B071) 11/38 [58] Field of Search 209/363, 364, 332,

FOREIGN PATENTS OR APPLICATIONS 30,163 12/1897 Great Britain 209/366 Primary Examiner-Frank W. Lutter Assistant Examiner-Robert Halper AttorneyMarechal, Biebel, French and Bugg [57] ABSTRACT A chip screen for separating over and undersized wood segments from the desired size chips incorporates a screen plate assembly which is mounted on a supporting base by means of corner bearings attached to the periphery rather than the bottom of the assembly, thereby lowering the center of gravity of the unit and providing a more stable unit. An eccentrically mounted stub shaft engages a socket on the bottom of the screen plate assembly to impart a circular translational movement to the screen plate assembly. The socket is made oversized and a resilient insert interposed between opposing surfaces of the socket and stub shaft so that the shaft may be centered with respect to the corner bearings without radial constraint and undue tensile and compressive stresses thereby avoided. A pair of counterweights are attached to the drive shaft and positioned longitudinally and circumferentially thereof such that the centrifugal forces generated by the counterweights and the screen plate assembly produce a dynamically balanced system of forces on the machine as a whole.

5 Claims, 11 Drawing Figures [56] References Cited UNITED STATES PATENTS 2,723,753 11/1955 Cook 209/332 131,919 10/1872 Wernz 209/332 X 555,461 2/1896 Morrison 209/332 650,693 5/1900 Shaler 209/332 X 681,254 8/1901 Northway 209/366 X 1,188,903 6/1916 Cornwall 209/366.5 X 1,730,435 10/1929 Munro 209/326 2,058,833 10/1936 Schneider.... 209/326 2,130,834 9/1938 Berry 209/415 2,230,316 2/1941 Wolz 209/326 X 2,705,561 4/1955 Denovan l 209/332 3,101,315 8/1963 Denovan 209/332 403,989 5/1889 Coxe 209/332 3,160,584 12/1964 Munter 209/415 X mimosa 4 am 3 3.75s;407

7O Fl G '1 N INVE/VTUR ELMER CHRISTENSEN A TTOR/VE Y PATENTEU SE? 4 I975 SNEEI 2 -(IF 3 FIG-7 VIBRATORY SCREENING WITH A PERIPHERAL SUPPORT BASE BACKGROUND OF THE INVENTION Vibrating screens are used in sorting operations in a number of industries. In the papermaking process, for example, oversized chips, pin chips, fines and the like may be separated from wood chips of a desired size by feeding the chips to a series of inclined screens of a proper mesh vibrating with a circular translational movement to separate oversized chips from those which are of the desired size and smaller. As the latter pass through the first screen they are received on a second screen which is of a mesh such that the chips of the desired size and retained while everything smaller passes through. Each of the three fractions, the oversized chips, the chips of the proper size and the smaller particles, are continuously withdrawn from the screening apparatus, with the correctly sized chips being fed into the papermaking process, the oversized chips being directed to a rechipper and the pin chips and the like utilized, for example, as fuel In similar operations a two sort system having one screenplate is used to separate oversize chips from acceptable chips, while in a four sort operation using three screen plates, oversize chips and fines and sawdust are separated from two sizes of acceptable chips.

Regardless of the number of screen plates used, however, in conventional practice, movement of the screen plate assembly containing the screens is obtained by mounting the assembly on a supporting base by means of eccentric corner bearings which extend from the bottom of the screen plate assembly to the base and permit a circular translational motion of the assembly with respect to the base. A rotatable drive shaft is mounted on thesupporting base, and a stub shaft, eccentrically positioned with respect to the longitudinal axis of the drive shaft, extends from the drive shaft to the bottom of the screen plate assembly so that upon rotation of the drive shaft the screen plate assembly moves with a circular translational motion with respect to the supporting base. To counterbalance the stresses imposed on the apparatus by the eccentric mounting of the screen plate assembly, a counterweight is usually positioned on the drive shaft on the side thereof opposite the side on which the stub shaft is offset with respect to the longitudinal axis of the drive shaft.

As noted, sorting apparatus of the type described above are used extensively in a variety of industries. However, certain disadvantages which inherently result with apparatus of this type have long been recognized. For example, the stub shaft extending between the drive shaft and the bottom of the screen plate assembly is snugly received in a socket, which may be either on the drive shaft or on the bottom of the screen plate assembly. As a result, it is necessary that the eccentricity of each of the corner bearings be precisely matched with the eccentricity of the stub shaft with respect to the longitudinal-axis of the drive shaft or the bottom of the screen plate assembly will be subjected continuously to compressive and tensile stresses with each revolution of the drive shaft.

It will also be seen that with movement of the screen plate assembly with respect to the base and attached thereto by means of the comer bearings extending from the bottom of the assembly to this supporting base, there is a tendency toward instability. Thus, each corner bearing experiences a cycling vertical load from the moving screen plate assembly. This load varies about a means value equal to the static bearing load and its amplitude is proportional to the height of the center of gravity of the assembly above the bearing level and inversely proportional to the longitudinal bearing spacing times the transverse bearing spacing or, the area of the square or rectangle defined by a line extending between the four bearings. Where the corner bearings are underneath the pan, resulting in a relatively high center of gravity and narrow bearing spacing, the variation in the vertical bearing load, the cycling load, becomes so great that a slight overload of chips on the screen may cause pounding in the bearings.

Additionally, while the mounting of a counterweight on the drive shaft on the side thereof opposite the direction in which the stub shaft is offset with respect to the longitudinal axis of the drive shaft does provide static balancing of the moving parts, the counterweight and the centrifugal force imposed on the screen plate assembly actually set up a force couple, resulting in a dynamically unbalanced system. It will be further noted that where the stub shaft is snugly received in the mounting socket the centrifugal forces generated by the orbiting screen plate assembly will impose a force on the drive shaft which, in actual practice, it will be almost impossible to calculate.

SUMMARY OF THE INVENTION It has been discovered that the stub shaft exerts only a pushing force on the wall of the socket in which it is received in a direction normal to a radius of the drive shaft, and it is therefore unnecessary for the stub shaft to be snugly received in the socket. Instead, the socket is deliberately made oversized to permit the stub shaft to be correctly positioned within the socket with respect to the eccentricity of the corner bearings and thereby obviate the alternating compressive and tensile forces which would ordinarily develop in the bottom of the screen plate assembly.

While theoretically, there will be no relative movement between the contacting faces of the stub shaft and the socket after initial startup of the unit, in actual practice there is a slight amount of sliding motion between these two surfaces. This results because in actual practice there will be some deflection of the supporting base at the corner bearings due to centrifugal forces which will permit the screen plate assembly to move relative to the drive shaft. Additionally, if the plane of motion of the screen plate assembly is not strictly perpendicular to the center line of the drive the socket in the screen plate assembly will oscillate up and down relative to the stub shaft. Further, if the screen plate assembly is not located correctly relative to the drive stud an oscillating motion parallel to the contacting surfaces will take place. Since the total displacement resulting from all of the above factors will only be on the order of a few hundredths of an inch, a pad of a resilient material may be inserted between the driving surfaces of the shaft and the socket to provide, in effect, an elastomeric lubrication.

The corner bearings mounting the screen plate assembly on the supporting base are attached to the screen plate assembly around the periphery rather than at the bottom thereof.,This lowers the screen plate assembly, and hence, the center of gravity of the entire unit, and improves the overall stability of the unit. The

corner bearings themselves are each fonned with separate bearing components for resisting vertical and horizontal loads. This reduces the overall size of the bearings and permits them to be kept compact. Additionally, the bearings are constructed such that some misalignment of the bearing pins can be tolerated, thereby eliminating the necessity of machining the frame supporting the screen plate assembly. It will also be seen that the radial loads are transmitted through the horizontal center plane of the bearings resisting horizontal loads to avoid movement loads on these bearings.

In order to counteract the problem of dynamic unbalance which generally prevails in sorting apparatus of the type described, a second counterweight is mounted on the drive shaft adjacent the lower end thereof and on the same side of the drive shaft as the stub shaft. By correctly sizing the counterweights and positioning them on the drive shaft with respect to the counterweights and positioning them on the drive shaft with respect to the effective position of the forces generated by the screen plate assembly, a dynamically, as well as statically, balanced system of forces results, not only improving the stability of the unit but reducing the effect of the forces acting on the system.

It will also be noted that since the stub shaft only contacts the receiving socket on one plane surface, the centrifugal forces from the screen plate assembly cannot directly affect the drive shaft since they are parallel to the contacting surfaces of the stub shaft and the socket and hence, cannot be transmitted to the drive shaft. Instead the centrifugal forces caused by the oscillating screen plate assembly go through the corner bearings and are balanced out with the counteracting forces from the counterweights on the drive shaft.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of sorting apparatus constructed according to the present invention;

FIG. 2 is a side elevational view of the apparatus of FIG. 1;

FIG. 3 is an end elevational view of the apparatus of FIGS. 1 and 2;

FIG. 4 is an enlarged plan view of one of the corner bearings of the assembly;

FIG. 5 is a cross-sectional view of the bearing assembly of FIG. 4;

FIG. 6 is an enlarged view of a portion of the structure of FIG. 5;

FIG. 7 is a cross sectional view through the stub shaft receiving socket on the screen plate assembly;

FIG. 8 is an elevational view, partly in cross section, showing the system of counterweights utilized in the invention;

FIG. 9 is a bottom plan veiw, partly in section, showing the screen plate assembly socket and stub shaft;

FIG. 10 is an elevational view of the stub shaft and socket, partly in section; and

FIG. 11 is a force diagram illustrating the principles of dynamic balance according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As noted above, one type of sorting apparatus in which the present invention finds utility is in a wood chip screen using one or more screen plates. As seen in FIGS. 1 through 3 of the drawings, apparatus of this type may comprise a supporting base 10 of a substantially H-shape and formed of a pair of spaced parallel side members 11 and 12 and cross beams 13 and 14. A motor 15 is mounted on a medial portion of cross beam 13, and a pulley 16 carried by the upper end of the drive shaft of the motor 15 has trained thereabout an endless belt 17.

A screen plate assembly 20, which is shown for purposes of illustration as a three sort, two screen system includes a first screen section 21 terminating in a spout 22 which projects outwardly of the screen section and is provided with converging side walls 23 and 24 and a bottom wall 25. A second screen section 26 extends beneath the screen 21 and receives all the material, for example, correctly sized chips, pin chips and fines, that passes through the screen 21. Ordinarily, the lower screen plate is divided into three equal sections with a step between each, which tumbles the chips to help separate fines and sawdust. Screen 26 is of the proper size mesh to permit the desired sized chips to be retained while pin chips, fines, etc. pass through and are collected separately. It will be understood, of course, that the number of screen plates used is immaterial for present purposes since the present invention finds utility in a variety of systems of different sizes.

As seen in FIG. 8, a drive shaft 30 is rotatably mounted in the supporting base 10, a portion of which is shown in phantom lines, by means of the mounting bracket 31. Shaft 30 is journaled in the mounting bracket 31 by means of bearings 32 and 33, which are maintained in position by means of the tube spacer 34 and retainers 35 and 36. Preferably, a lubrication fitting 37 is mounted on tubing 38 leading to the drive shaft bearing assembly to permit ready lubrication thereof.

With reference to FIGS. 6 through 9 of the drawings, it will be seen that the drive shaft 30 terminates in a stub shaft 39 eccentrically mounted thereon and received in a socket 40 defined in an insert 41. Insert 41 is rotatably mounted in the housing 42 by means of the bearing 43 and snap ring 44. The bearing 43 is retained in the housing by means of retaining ring 45 attached to the housing by means of bolts 46. The bearing housing is in turn attached to frame members 47 on the bottom of the screen plate assembly 20 by means of bolts 48. A fitting 49 may be provided mounted on the housing 42 for lubricating the bearing 43.

The stub shaft 39, as best seen in FIG. 8, is provided with a planar surface 50, a pair of planar side surfaces 51 extending at right angles to the surface 50 and a curved surface 52 interconnecting the two side surfaces. The socket 40 is of substantially the same configuration as that of the stub shaft but is of appreciably greater internal dimensions than the external dimensions of the stub shaft. Thus, the socket 40 is provided with a planar surface 50', a pair of planar side surfaces 51 and a curved surface 52'.

As noted above, there will, for a variety of reasons, be some small relative movement, on the order of a few hundredths of an inch, between the surface 50 and 50' during operation of the unit. In order to prevent wear between these surfaces, a resilient insert 53, which may consist of a pad of rubber or the like, is interposed between the surfaces 50 and 50'. The rubber or other material is selected such that there will be no relative movement between it and the surface with which it is in contact, and thus, the pad 53 will be deformed upon relative movement between the surfaces 50 and 50' and eliminate the need for lubrication between these two surfaces, As seen in FIGS. 7 and 10, the pad 53 may be retained in place by a lower plate member 54 and an upper plate member 55 secured to the insert 41 by means of a bolt or the like 56.

As seen in FIG. 8, the shaft 30 has mounted adjacent its upper end a sheave 60 which receives the belt 17 trained about the pulley 16 of the motor 15. In addition to receiving the belt 17, the sheave 60 also serves as means for mounting the counterweights 61 on the shaft 30 on the side thereof opposite the stub shaft 40. The counterweights 61 may be attached to the sheave 60 by means of bolts 62 passing through the web 63 of the sheave. Adjacent its lower end, the shaft 30 is provided with additional counterweight means 65 mounted on the shaft on the same side thereof as the stub shaft 40 by means of a split hub 66 and bolts 67. Although both counterweights are described as being bolted in place, it will be apparent that any attaching means could be utilized, including casting the counterweights in place.

Turning now to FIGS. 1 through 60f the drawings, it will be seen that the screen plate assembly 20 is attached to the supporting base by means of corner bearings 70 attached to the periphery of the screen plate assembly by means of brackets 71 and to the supporting base by means of bearing housings 72. Each of the brackets may be of the split ring type having a pair of outwardly projecting flanges 73 which may be drawn toward each other by means of the bolts or the like 74. The brackets engage bearing pins 75 which are provided with lock washers 76 and lock nuts 77 to provide initial vertical adjustment of the screen plate assembly with respect to the corner bearing housings 72. The lower end of each of the pins 75 is reduced and rotatably mounted by means of bearings 78 and 79 is an eccentric insert 80. Insert 80 is in turn rotatably mounted in the bearing housing 72 by means of the bearings 81 retained in place by the bearing covers 82. It is also desirable to provide a grease fitting 83 and a bearing seal 84 to maintain proper lubrication of the comer bearlngs.

As better seen in FIG. 6 of the drawings, the lower end 90 of bearing pin 75 is received within the bearing 79, which includes an upper race 92, a lower race 94 and a plurality of balls 96. The upper race 92 is fixed to the pin 75 and the lower end 97 of lower race 94 is convexly curved to slidably. rest on the concavely curved upper surface of the washer 98 seated in the socket 100 of the eccentric insert 80. A spacer ring 102 also surrounds the lower end of pin 75 and spaces the spherical roller bearing 78 from the spherical ball hearing 79. Bearing 78 consists of opposed inner and outer races 104 and 196 and a plurality of rollers 108 interposed therebetween and it will be noted, as indicated at 109 in FIG. 5 of the drawings, that the center lines of the roller bearings 108 and the balls 110 of bearing 81 are aligned,

In operation, the motor 15 rotates the pulley 16 and,

stub shaft on the drive shaft, their positions could be reversed with the stub shaft extending from the screen plate assembly and the socket formed in the end of the drive shaft.

Regardless of this, however, because the socket 40 is made oversized with respect to the stub shaft 39 the stub shaft need not be precisely positioned correctly with respect to the eccentricity of the comer bearings since the resilient pad 53 provides, in effect, an elastomeric lubrication between the surfaces 50 and 50 as these two surfaces are displaced with respect to each other. The need for close machining tolerances is, therefore, eliminated and the stub shaft need only be positioned within the socket with an exactness corresponding to the shear strain working limits of the pad 53.

As noted above, although the internal dimensions of the socket 40 are appreciably greater than the external dimensions of the stub shaft 39, both members are of substantially the same configuration the only variation being the small semicircular cutouts formed on the stub shaft socket for manufacturing reasons. With each of the members thus formed of the same irregular configuration, it is impossible inadvertently to attach the stub shaft in a reversed position.

With the corner bearings attached to the periphery of the screen plate assembly 20 rather than the bottom thereof, the center of gravity of the screen plate assembly, and hence of the entire screen apparatus, is appreciably lower than in conventional structures of this type, and this, in conjunction with the outwardly extending supporting base 10, greatly improves the stability of the unit. As the screen plate assembly is driven by the motor 15 it will be seen that all radial loads on the bearing 81 are transmitted in a horizontal plane through the center of the bearings, as indicated by phantom line 109, avoiding moment loads on the bearmg.

With regard to bearings 78 and 79 it will be seen that their combination provides two results. First, the vertical and horizontal loads are each carried by separate bearings with bearing 78 taking horizontal loads and bearing 79 taking vertical loads. A spherical roller bearing of a size sufficient to resist both vertical and horizontal loads would be rather large. Additionally, since this bearing would be carried within the socket 100 of the eccentric 80, this would increase the size of the eccentric and the inside diameter of the bearing 81. Any increase in the inside diameter of bearing 81 however, will cause a sharp increase in the outside diameter, height and cost of bearing 81 at a higher rate than, and disproportionate to, the rate of increase of the inside diameter thereof. As a result this has often been a critical factor in the size of the screens being designed.

Secondly, it will be seen that it is not necessary for the bearing pin to be perfectly aligned, since the spherical bearing arrangement will permit some misalignment. Thus, if the pin is offset somewhat from the vertical, the convexly curved lower end of race 94 may slide on the concavely curved upper surface of washer 98, while the sperhical roller bearing races 104 and 106 shift somewhat with respect to each other. This eliminates the need for machining the supporting frame, which is very difficult in large screens, and close tolerance fabrication, both of which increase manufacturing costs.

As noted above, it is conventional to mount a counterweight on the screening apparatus drive shaft on the side of a drive shaft opposite the eccentric mount of the stub shaft. Upon rotation of the drive shaft, however, the screen plate assembly and the counterweight will generate centrifigal forces which will result in a dynamically unbalanced system.

By means of the present invention however, this dynamic unbalance is avoided so that the sum of the moments taken about any point, for example, any point on the drive shaft, during operation of the screening unit is equal to zero. Thus as seen in FIGS. 8 and 11, the forces acting upon the system during operation of the screen apparatus are the centrifugal forces generated by the mass of the screen plate assembly (m the mass of the counterweight 61 (m and the mass of the counterweight 65 (m The force exerted by the screen plate assembly, the counterweight 61 and the counterweight 65 will be proportional, in each case, to the mass multiplied by the radial distance from the axis of the shaft 30 to the effective point at which the mass acts. These radial distances are given for the masses m m and m as e, r and r respectively. It will thus be seen that in order to have a statically balanced system of forces acting on the shaft 30 m r 1 m e +m r Further, to obtain dynamic balance of the system, the sum of the moments about any point on the shaft must equal zero. Thus taking the moments of the forces about point A, for convenience, it will be seen that the centrifugal force m e acting through the lever arm X must equal the centrifugal force m r acting through the lever arm Y, or

m eX m r Y This requirement, combined with requirement for static balance supra, gives In an actual installation, the values of e, m m m r r X and Y are found to be as follows:

e 1.25 inches m 5,442 pounds m 876 pounds m 414 pounds X 13.58 inches Y 20.32 inches r 13.30 inches r inches With these values, not only was the system statically balanced but it was dynamically balanced as well.

From the above it will be apparent that the present invention has overcome the problems of instability, dynamic imbalance and undue, alternating compressive and tensile stresses which usually lead to fatique failure.

While the form of apparatus herein described constitutes a preferred embodiment of the invention, it is to be understood that the invention is not limited to this precise form of apparatus, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.

We claim:

1. Screening apparatus comprising a supporting base, a pan assembly including screen means positioned above said supporting base, portions of said supporting base extending outwardly beyond peripheral portions of said pan assembly, bearing means on said outwardly extending portions mounting said pan assembly on said supporting base for eccentric oscillatory motion with respect to said base, each of said bearing means including a bearing pin, means mounting said bearing pin on said peripheral portion of said pan assembly, an eccentric insert, outer bearing means journalling said insert in one of said outwardly extending portions, means defining a bearing socket in said insert offset from the center thereof and receiving one end of said pin, vertical bearing means surrounding a portion of said one end of said pin and resisting loads primarily in a vertical direction, and horizontal bearing means surrounding a second portion of said one end of said pin and resisting loads primarily in a horizontal direction, a drive shaft rotatably mounted on said base, means for rotating said drive shaft about its longitudinal axis, a stub shaft extending between said drive shaft and said pan assembly with the longitudinal axis thereof extending in spaced parallel relationship to said drive shaft longitudinal axis, and a stub shaft socket offset with respect to said longitudinal axis of said drive shaft and receiving said stub shaft.

2. The apparatus of claim 1 further comprising means defining a concave bearing seat in said bearing socket, said vertical bearing having a convex surface slidably engaging said concave seat.

3. The apparatus of claim 1 wherein said horizontal bearing is a spherical roller bearing, and the rollers thereof extending substantially parallel to said pin.

4. The apparatus of claim 1 wherein the centerlines of said outer and horizontal bearings are substantially aligned.

5. A chip screen comprising a pan assembly having bearing brackets mounted on side walls thereof and extending outwardly from said side walls, a substantially l-l-shaped supporting base mounted beneath said pan assembly with distal portions of said base extending outwardly beneath said bearing brackets, bearings mounted on each of said distal portions and having bearing pins received in said brackets and joumaled for circular translational movement about axes parallel to said pins, a drive shaft rotatably mounted on said base in a medial portion thereof, said drive shaft having a stub shaft extending from the upper end of said drive shaft and offset with respect to the longitudinal axis thereof, said stub shaft having a planar surface and a curved surface opposite said planar surface with said stub shaft being loosely received in a socket of the same configuration but appreciably larger dimensions mounted on a lower surface of said pan assembly, a sheave carrying a counterweight mounted on said drive shaft adjacent said upper end thereof on the side of said drive shaft opposite said stub shaft, and a second counterweight mounted on said drive shaft adjacent the lower end thereof, said counterweight being of such mass and positioned with respect to each other and said pan assembly such that the centrifugal forces generated by said counterweights and said pan assembly are dynamically balanced.

I i i UNITED STATES 'PA'IIINT OFFICE CERTIFICATE OF CORRECTION V Patent No. 3,756, 407 Dated September 4, 1973 Inventofls) Elmer Christensen It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3 line 11, "movement" should be-- moment. I

Column 3, lines l8, l9 and 20, "the centerweights and positioning them on the drive shaft with v respect to" should bel'omitted.

t column 5, line '35, "is" should b'e-in--.

. Column 6, line 63, "sperihical" should-be spherical--.

Column 7, n line.5l, "11.50" is omitted before inches;

7 Column 8, line 34, "extending" should be.- extend--..

Signed and sealed this- 25th day of December 1973.

(SEAL) Attest: I

EDWARD M.FLETCHER,JR. t RENE D. TEGTMEYER Attesting Officer I Acting Commissioner of Patents 

1. Screening apparatus comprising a supporting base, a pan assembly including screen means positioned above said supporting base, portions of said supporting base extending outwardly beyond peripheral portions of said pan assembly, bearing means on said outwardly extending portions mounting said pan assembly on said supporting base for eccentric oscillatory motion with respect to said base, each of said bearing means including a bearing pin, means mounting said bearing pin on said peripheral portion of said pan assembly, an eccentric insert, outer bearing means journalling said insert in one of said outwardly extending portions, means defining a bearing socket in said insert offset from the center thereof and receiving one end of said pin, vertical bearing means surrounding a portion of said one end of said pin and resisting loads primarily in a vertical direction, and horizontal bearing means surrounding a second portion of said one end of said pin and resisting loads primarily in a horizontal direction, a drive shaft rotatably mounted on said base, means for rotating said drive shaft about its longitudinal axis, a stub shaft extending between said drive shaft and said pan assembly with the longitudinal axis thereof extending in spaced parallel relationship to said drive shaft longitudinal axis, and a stub shaft socket offset with respect to said longitudinal axis of said drive shaft and receiving said stub shaft.
 2. The apparatus of claim 1 further comprising means defining a concave bearing seat in said bearing socket, said vertical bearing having a convex surface slidably engaging said concave seat.
 3. The apparatus of claim 1 wherein said horizontal bearing is a spherical roller bearing, and the rollers thereof extending substantially parallel to said pin.
 4. The apparatus of claim 1 wherein the centerlines of said outer and horizontal bearings are substantially aligned.
 5. A chip screen comprising a pan assembly having bearing brackets mounted on side walls thereof and extending outwardly from said side walls, a substantially H-shaped supporting base mounted beneath said pan assembly with distal portioNs of said base extending outwardly beneath said bearing brackets, bearings mounted on each of said distal portions and having bearing pins received in said brackets and journaled for circular translational movement about axes parallel to said pins, a drive shaft rotatably mounted on said base in a medial portion thereof, said drive shaft having a stub shaft extending from the upper end of said drive shaft and offset with respect to the longitudinal axis thereof, said stub shaft having a planar surface and a curved surface opposite said planar surface with said stub shaft being loosely received in a socket of the same configuration but appreciably larger dimensions mounted on a lower surface of said pan assembly, a sheave carrying a counterweight mounted on said drive shaft adjacent said upper end thereof on the side of said drive shaft opposite said stub shaft, and a second counterweight mounted on said drive shaft adjacent the lower end thereof, said counterweight being of such mass and positioned with respect to each other and said pan assembly such that the centrifugal forces generated by said counterweights and said pan assembly are dynamically balanced. 